This invention relates to a method for calculating gain correction factors in a digital imaging system and more particularly to a method for reducing fixed pattern structural noise in gain equalization factor calculations.
Large size image detectors are well known in the art as exemplified by U.S. Pat. No. 5,773,832, issued Jun. 30, 1998to Sayed et al., U.S. Pat. No. 5,254,480, issued Oct. 19, 1993 to Nang T. Tran, U.S. Pat. No. 5,315,101, issued May 24, 1994 to Hughes et al. or U.S. Pat. No. 5,381,014, issued Jan. 10, 1995 to Jeromin et al.
These detectors typically comprise arrays of millions of individual sensors which output an electrical signal representing the radiation exposure of the sensor, and whose combined output represents the image projected by the exposing radiation onto the detector. The output of each sensor represents one picture element (pixel) of the captured image.
The current state of the art in manufacturing such detectors, while extremely advanced, has not been able to produce detectors in which the individual sensors respond exactly the same to an exposure level. Thus when a detector is illuminated with uniform intensity radiation, the output of the individual sensors is not, as one would expect, uniform. To the contrary, the output varies from pixel to pixel. In addition the gain of each amplifier in the readout system also varies between amplifiers.
The art has addressed this problem by calculating a gain correction coefficient for each pixel such that when the output of each pixel is processed (usually multiplied) by this coefficient, the output of all pixels is uniform for a uniform exposure. These individual gain correction coefficients are usually stored in a table (i.e. a Look Up Table or LUT) and are used to correct the values obtained for each pixel when the detector is subsequently used to detect an image.
The problem with this approach, however, is that the output non-uniformity in the uniform exposure signal used to calculate the gain correction coefficients is not solely due to sensors output differences. Before the signal can be digitized and made available for processing, it undergoes detection and amplification in analog form followed by A to D conversion in various electronic components. Thus the output signal contains, in addition to the actual sensor output non-uniformities, noise from the electronic components handling the signal, including the amplification and digitization stages. This noise is random and not expected to repeat between exposures. Thus correcting the output of subsequent images with a coefficient calculated for a single exposure introduces noise which distorts the image.
Another source of error in the calculation of the gain correction coefficients is the uniform illumination used to obtain the uniform (also known as flat field) exposure of the detector in the first place. It is extremely hard to create a perfectly uniform radiation field over a large surface, especially when the radiation is x-ray radiation and the detector is a large 14xc3x9717 inch detector typically used in digital radiography for medical diagnostic purposes. While this problem could be corrected out, unfortunately the illumination also varies between exposures adding to the source of unpredictable noise.
In this application, we will refer to the above sources of non-uniformity in the sensors output collectively as structural noise.
The solution adopted by the prior art has been to generate a multiplicity of gain correction factors from a plurality of distinct flat field exposures of the detector and average the gain factors for each pixel to obtain an average gain factor thus eliminating to a large degree the random noise errors.
While this process is effective, it is also time consuming and therefore expensive. This is particularly undesirable because the gain factors should be checked often to correct the LUT to compensate for aging of the sensors or for changes in the environment where the detector operates.
There is, therefore, still a need to develop a process for calculating an accurate gain factor for each pixel of a detector having a plurality of sensors quickly and easily, without requiring multiple exposures of the sensor.
In accordance with this invention, the aforementioned problems are alleviated through a method for developing a set of corrected digital gain correction coefficients for use in correcting digital values representing an image captured by a detector comprising a plurality of individual sensors, each sensor representing a pixel, the method comprising:
A) exposing the detector to radiation having a substantially uniform intensity distribution over all sensors to obtain an initial set of pixel values;
B) developing a first set of gain correction coefficients; and
C) applying a smoothing filter to said first set of gain correction coefficients to obtain a set of corrected gain correction coefficients.
Applying a smoothing filter may comprise identifying a filter window comprising a plurality of pixels surrounding and including a target pixel and calculating a corrected gain correction coefficient using only the gain correction coefficients in said filter window.
The calculation of the corrected gain correction coefficient for the target pixel may, for example, comprise comparing the coefficient values of the first set of gain correction coefficients for the plurality of pixels in the filter window, and obtaining a mean value which is used as the corrected gain correction coefficient for the target sensor.
The present invention also provides a method for correcting an image for structural noise in the output of a plurality of radiation detection sensors forming a detector, each of said sensors representing a pixel and comprising a radiation detection conversion element, and signal processing electronic circuitry for producing a digital output representing the radiation exposure of each of said sensors, the method comprising in the following order.
1. Exposing the plurality of sensors to a substantially uniform intensity radiation;
2. Obtaining a pixel digital output value from each of said plurality of sensors;
3. Calculating a median value of the of all of said pixel digital output values;
4. Calculating for each pixel a first gain coefficient equal to the ratio of the pixel digital output value to the median value;
5. Applying a smoothing filter to said first gain correction coefficients to obtain a set of corrected gain correction coefficients.
6. Storing the corrected gain correction coefficients in a look up table and
7. Using said corrected gain correction coefficients to correct an image captured by the detector.